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  Thick film conductor composition(s) and processing technology thereof for use in multilayer electronic circuits and devicesUSPTO Application #: 20070113952Title: Thick film conductor composition(s) and processing technology thereof for use in multilayer electronic circuits and devices Abstract: The invention relates to thick film conductor compositions which are useful in application to both via-fill and/or line conductors to manufacture of Low Temperature Co-fireable Ceramic (LTCC) devices and other Multilayer Interconnect (MLI) ceramic composite circuits such as Photosensitive Tape On Substrates (PTOS); gold, silver and mixed metal multilayer circuits and devices. The invention is useful for forming microwave and other high frequency circuit components selected from the group comprising: antenna, filters, baluns, beam former, I/O's, couplers, via feedthroughs, EM coupled feedthroughs, wirebond connection, and transmission lines. (end of abstract)
Agent: E I Du Pont De Nemours And Company Legal Patent Records Center - Wilmington, DE, US Inventors: Kumaran Manikantan Nair, Mark Frederick McCombs USPTO Applicaton #: 20070113952 - Class: 156089170 (USPTO) Related Patent Categories: Adhesive Bonding And Miscellaneous Chemical Manufacture, Methods, Surface Bonding And/or Assembly Therefor, With Vitrification Or Firing Ceramic Material, Forming Electrical Article Or Component Thereof, Elemental Metal Or Alloy Containing, Silver Containing The Patent Description & Claims data below is from USPTO Patent Application 20070113952. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD OF THE INVENTION [0001] The invention relates to thick film conductor compositions which are useful in application to both via-fill and/or line conductors to manufacture of Low Temperature Co-fireable Ceramic (LTCC) devices and other Multilayer Interconnect (MLI) ceramic composite circuits such as Photosensitive Tape On Substrates (PTOS); gold, silver and mixed metal multilayer circuits and devices, as well as the cost-effective manufacturing of LTCC, PTOS, MLI circuits and devices. Compositions are single printable to reduce the cost of printing and processing of both line conductors and via-fill conductors together or singly. The invention also relates to printing and processing technology of high solid-containing and low viscosity via-fill thick film compositions to make multilayer devices comprising a large number of layers and thicker tapes. TECHNICAL BACKGROUND OF THE INVENTION [0002] Thick film conductors serve as electrical interconnections between resistors, capacitors, inductors, integrated circuits etc. Besides, in multilayer devices and circuits, these conductors interconnect between layers of conductor lines through vias built into the devices. In other words, tape-casted ceramic green sheets for LTCC devices are punched, screened with appropriate conductor lines, stacked and laminated prior to firing. Punching involves mechanical perforation of vias through each layer which are filled with via-fill conductor formulations to interconnect the circuitry between layers. Processing of PTOS circuits are described in a recent invention disclosure U.S. Provisional Patent Application No. 60/703,530, herein incorporated by reference (Attorney Docket No. EL-0570). In other words, interconnect devices are an electronic circuits and surface and buried sub systems of resistors, capacitors, inductors etc that are connected both by electrically and mechanically, "green systems", electronic systems contain very little or no known toxic materials, is a new goal for environmentally safer to the public-at-large and furthermore the public desired such systems. [0003] Similar to other thick film materials, thick film conductors and via-fills are comprised of an active (conductive) metal and inorganic binders, both of which are in finely divided form and are dispersed in an organic vehicle. The conductive phase is ordinarily gold, palladium, silver, platinum or alloy thereof, the choice of which depends upon the particular performance characteristics which are sought, e.g. resistivity, solderability, solder leach resistance, bondability, adhesion, migration resistance and the like. In multilayer devices, internal conductor lines and via conductors additional performance characteristics which are sought, e.g. minimization of conductor line "sinking" into the top and bottom dielectric layers on firing, resistivity variation on repeated firing, interface connectivity of line conductor to that of via-fill conductor, interface bonding of via-fill conductor to that of surrounding ceramic materials. [0004] Thick film techniques are contrasted with thin film techniques which involve deposition of particles by evaporation or sputtering with or without vacuum. Thick film techniques are well known to those skilled in the art. [0005] In addition to the proper level of conductivity and other properties listed above, there are many secondary properties which must also be present such as, wire bondability, good adhesion to both ceramic and thick films, solderability and compactibility to other thick films, both surface and buried, long-term stability without little or less properties degradation. [0006] As would be expected, one critical variable in the technology of thick film conductors for use in the multilayer interconnect devices is the resistivity variation by interaction with surrounding ceramics. Of particular importance in this regard has been the incorporation of high-melting refractory glasses and glasses have little or no miscibility with the remnant glasses present in the surrounding ceramics. Furthermore, additional incorporation of metal oxide and non-metal oxide binder materials in the composition increases the densification of the conductor composites and/or control the growth of crystalline materials into the conductor composites which would change the resistivity of the composites, a non-desirable result. [0007] Additionally, the second most critical variable in the technology used in the MLI devices is the interface connection of via-fill buried within and the line conductor. Due to the differences in the solids and printing conditions which are needed to fill the vias completely and to connect vias to the line conductors result in conductor-via interface stress development and significant delaminations and microstructure variations at the interface which result in conductivity differences, an unwanted result. [0008] As would be expected, the third most critical variable in the state-of-the-art thick film technology used in the MLI devices is the printing differences needed to fill vias and line conductors, due to the significant viscosity differences of the formulations. Typical via-fill conductor formulation viscosity is in the range of 1500-7000 or more PaS and that of line conductors is typically in the range of 150-300 PaS. These viscosity differences are critical to fill vias and fast printing of line conductor formulations. Lower viscosity via-fill formulations could "flow out of the bottom of the tape" and show "peaking"; and high viscosity formulations need more than one print to fill the vias. Many high viscosity formulations also show "posting" of the vias. On the other hand, line conductor formulation viscosity should be kept low in order to print connective lines without line-break, thinner lines and print the formulation fast enough for better productivity. [0009] The fourth most critical variable is the processing of multilayer interconnect laminate without delamination particularly thicker laminates containing higher number of layers and thicker Green tapes up to 10 mils or higher. These green parts contain large amount of organics. During the heat processing, resins and polymers present will "depolymerize" to monomers, dimers etc. which have higher vapor pressure than the parent polymer, leaves the system at the early stages of the processing. The residual organic components decompose to carbon containing species later in the process. [0010] The temperature profile needed to get rid of all the organics before softening of the glasses present in the tape depends on: quantity and quality of the organics, rate of heating, temperature, firing atmospheric conditions etc. Variations of one or more of the above components result in delaminated of layers, via-to-tape side wall separation, via-to-line conductor separation and finally yield loss. BRIEF DESCRIPTION OF THE FIGURES [0011] FIGS. 1a & 1b: FIG. 1a represents the structural formula of ethylcellulose. FIG. 1b represents the structural formula of ethylcellulose with complete (54.88%) ethoxyl substitution. [0012] FIG. 2: "Daisy chain test pattern" consisting of 308 via and 5133 squares of conductors. Conductor line width 6 mils. Dashed line represents internal buried via connections and solid lines represent surface via connections. [0013] FIGS. 3 & 4: Resistivity variation as a function of number of refires. Two types of processing conditions were used. (1) "Double wet pass" filling both the vias and line conductors together (2) Fill the vias first and print the line conductor over it using the same composition. FIG. 3 refers to silver conductor on low loss tape 943-PX (E. I. du Pont de Nemours and Company) and FIG. 4 refers to gold conductors on 951-AT tape (E.I. du Pont de Nemours and Company). SUMMARY OF THE INVENTION [0014] The present invention relates to a thick film composition for use in low temperature co-fired ceramic circuits comprising, based on weight percent total thick film composition: [0015] (a) 30-96 weight percent finely divided particles selected from noble metals, alloys of noble metals and mixtures thereof; [0016] (b) one or more inorganic binders selected from: [0017] (1) 0.2-10 weight percent of one or more refractory glass compositions with a specific viscosity (log n) in the range of 6-7.6 at the firing temperature of said circuit; [0018] (2) 0.1-5 weight percent of an additional inorganic binder selected from (i) metal oxides, (ii) precursors of metal oxides; (iii) non-oxide borides; (iv) non-oxide silicides; and (v) mixtures thereof; and [0019] (3) mixtures thereof; dispersed in [0020] (c) 2-20 weight percent organic medium comprising ethyl cellulose with a specific ethoxyl content in the range of 45.0-51.5 and degree of substitution of ethoxyl groups per anhydroglucose unit in the range of 2.22-2.73. [0021] In a low temperature co-fired ceramic circuit, the above composition may be processed to sinter the glass, the metal powders and/or other sintering aids and to remove organic medium. In one embodiment the refractory glass composition comprises, based on weight percent total glass composition, 25-60% SiO2, 10-20% Al2O3, 10-15% B2O3, 5-25% CaO, and 1-5% balance other network modifying ions. [0022] The invention further comprises a method of forming a multilayer circuit comprising: [0023] a) forming a patterned array of vias in a plurality of layers of green tape; [0024] b) filling the vias in the green tape layer(s) of step (a) with a via-fill thick film composition; [0025] c) printing patterned thick film functional layers over a surface of any or all of the via-filled green tape layers of step (b); [0026] d) printing patterned layers of a surface thick film over the outermost surface of the Green tape layers of step (c); [0027] e) laminating the printed green tape layers of step (d) to form an assemblage comprising a plurality of unfired interconnected functional layers separated by unfired green tape; and [0028] f) cofiring the assemblage of step (e); and wherein one or more of said via-fill thick film composition, patterned thick film functional layers and surface thick film utilize the above composition. [0029] The invention further comprises: [0030] a) forming a patterned array of vias in a plurality of layers of green tape; [0031] b) filling the vias in the green tape layer(s) of step (a) with a via-fill thick film composition; [0032] c) printing patterned thick film functional layers over a surface of some or all of the via-filled Green tape layers of step (b); [0033] d) laminating the printed green tape layers of step (c) to form an assemblage comprising a plurality of unfired interconnected functional layers separated by unfired green tape; [0034] e) printing at least one patterned layer of surface thick film composition over the assemblage of step (d); and [0035] f) cofiring the assemblage and patterned layer(s) of step (e); and [0036] wherein one or more of said via-fill thick film composition, patterned thick film functional layers and surface thick film utilize the above composition. [0037] In several embodiments, the present invention relates to the methods above wherein said via-fill thick film composition and at least one of said thick film functional layer(s) utilize the composition of the present invention and wherein said via-fill thick film composition and thick film functional layer(s) are printed simultaneously in one single step. The invention also relates to the circuits formed by the above methods. DETAILED DESCRIPTION OF THE INVENTION [0038] The invention is therefore directed to a the processing of noble metallization with unique organic components to maintain low enough viscosity to single-print both conductor lines and via-fill conductors without posting or flowing out. The invention is further directed specifically to the processing of LTCC devices where printing over the tape backing such as mylar; in the case PTOS devices, fill the vias including the "undercutting" of the tape after exposure for the manufacture of printed conductors. Fired circuits have outstanding electrical properties on repeated firing of the devices. In particular, the invention is directed to a series of metal metallizations suitable for the manufacture of pure silver, gold and mixed metal electronic devices, which is admixture of: [0039] (a) 30-96 weight percent finely divided particles selected from noble metals, alloys of noble metals and mixtures thereof; [0040] (b) one or more inorganic binders selected from: [0041] (4) 0.2-10 weight percent of one or more refractory glass compositions with a specific viscosity (log n) in the range of 6-7.6 at the firing temperature of said circuit; [0042] (5) 0.1-5 weight percent of an additional inorganic binder selected from (i) metal oxides, (ii) precursors of metal oxides; (iii) non-oxide borides; (iv) non-oxide silicides; and (v) mixtures thereof; and [0043] (6) mixtures thereof; dispersed in [0044] (c) 2-20 weight percent organic medium comprising ethyl cellulose with a specific ethoxyl content in the range of 45.0-51.5 and degree of substitution of ethoxyl groups per anhydroglucose unit in the range of 2.22-2.73. 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